This invention relates to a process for fabricating compound semiconductor devices. More particularly, this invention relates to a process for fabricating compound semiconductor devices such as high electron mobility transistors (hereinafter referred to simply as "HEMT") that are suitable for use as low-noise amplifying devices operating in the microwave band.
A known process for fabricating a HEMT typically comprises the following steps. In the first step, as shown in FIG. 2a, a GaAs buffer (non-doped GaAs) 1 is overlaid successively with an n.sup.+ -AlGaAs layer 2 and an n.sup.+ -GaAs layer 3.
In the second step, as shown in FIG. 2(b), a drain electrode 8 and a source electrode 9 made of a metal establishing an ohmic contact (e.g. AuGe, Ni or Au) are formed on the n.sup.+ -GaAs layer 3 by conventional methods. For example, the drain electrode 8 and source electrode 9 may be formed by depositing metal by vacuum vapor deposition with a resist pattern having openings being used as a mask. The n.sup.+ -GaAs layer 3 is recessed at later stages to form a drain region 3a and a source region 3b of the HEMT as shown in FIG. 2(f).
In the third step, as shown in FIG. 2(c), a resist pattern 5 having a reversely tapered opening 4 is formed over the n.sup.+ GaAs layer 3 and the electrodes 8 and 9. The opening 4 is reversely tapered in order to facilitate the liftoff process in the sixth step to be described later.
In the fourth step, the part of the n.sup.+ -GaAs layer 3 that is exposed through the opening 4 is subjected to recess-etching, with the resist pattern 5 being used as a mask, whereby the n.sup.+ -AlGaAs layer 2 is made exposed as shown in FIG. 2(d). The recess etching is accomplished by wet etching with a commonly used etchant (e.g. a liquid mixture of H.sub.2 O.sub.2 and H.sub.2 SO.sub.4).
In the fifth step, with the resist pattern 5 being used as a mask, a gate metal 6 such as Al and W is deposited vertically by vacuum vapor deposition on the area of n.sup.+ -AlGaAs layer 2 that is exposed through the opening 4, whereby a gate electrode 7 is formed on the n.sup.+ -AlGaAs layer 2 (see FIG. 2(e)).
In the sixth step, the resist pattern 5 is removed so as to lift off the unwanted gate metal 6, whereby the n.sup.+ -GaAs layer 3 as well as the drain electrode 8 and source electrode 9 formed on that layer are made exposed, thus completing the process of HEMT fabrication (see FIG. 2(f)).
Generally speaking, the smaller the distance between a gate and source, the smaller the source resistance and hence the low noise characteristic of a HEMT is accordingly improved. As shown in FIG. 2(f), the distance between the gate electrode 7 and the drain region 3a of the device fabricated by the above process is equal to the distance between the gate electrode 7 and the source region 3b. Hence, if one wants to improve the low noise characteristic of the device by reducing the distance between the gate electrode 7 and the source region 3b, the distance between the gate electrode 7 and the drain region 3a will inevitably decrease, causing a substantial drop in the breakdown voltage between the gate and drain.
With a view to solving this problem, Unexamined Published Japanese Patent Application No. 21877/1988 proposed that the distance between the gate electrode and the drain region be reduced by oblique vacuum vapor deposition of a mask material. However, according to this method, the position of an opening in the mask is determined by the amount of the oblique deposition, so that it is very difficult to form the gate electrode at a desired position with high precision.
The most effective way to improve the low noise characteristic of a HEMT is to shorten the gate length, thereby reducing the gate capacitance (C.sub.gs) while increasing the transconductance (g.sub.m). In fact, however, a shorter gate length leads to a greater gate resistance, which may result in a deteriorated low noise characteristic.